The evolution of mobile communications had led to an extensive cellular infrastructure, which is integrally connected to the Public Switched Telephone Network (PSTN). Traditionally, cellular networks and the PSTN have supported voice communications, while packet networks have been predominantly used for data communications. In recent years, packet networks have been used to support voice communications. Since cellular networks and the PSTN dominate voice communications, there is a need to integrate packet networks, cellular networks, and the PSTN for voice communications, especially for evolving packet-based communication clients. There is a further need to allow packet-based communication clients to readily make and receive calls supported in part over the PSTN or cellular networks in an efficient manner.